The present invention relates to an evaporator comprising upwardly or vertically extending outer tubes having closed upper ends and being connected to a tube plate in a lower region of the evaporator. Inside these tubes there are other normally parallel tubes connected to another tube plate which is disposed below the said tube plate. The upper region of the evaporator is provided with means for feeding the material to be evaporated to the upper ends of the outer tubes so that the material flows down along the outer tubes forming a falling film on the outer surface of the outer tubes. An inlet and outlets are provided for the material delivering heat in a lower region of the evaporator.
A double-tube falling film evaporator is disclosed in German patent publication no. 737330. In the apparatus of this publication the material delivering heat is fed to a space under a lower tube plate from which it flows in an inner tube to the internal surface of an outer tube to which it delivers heat which is used for evaporating material flowing down along the external surface of the tube. One of the advantages of this construction is that the heat exchanger tube is connected to only one tube plate and thus is not subjected to stresses as the tubes in a conventional tube-type evaporator. Further, only one of the tube plates, viz. the upper one, has to face the pressure difference prevailing in the evaporator and not both the tube plates as in a conventional tube-type evaporator.